Abstract - We studied the effect of ageing on the mobility and sequestration of 14C-phenanthrene in an agricultural soil. We used autovial microcolumns incubated for a period ranging between 0 and 156 days. Our
results showed that a large proportion of radioactivity remained extractable
by methanol up to 64 days of incubation, amounting from 44.5 to 80.2% of initial radioactivity. Mineralisation of 14C-phenanthrene began tardily, after the decrease of mineralisation of
the soil organic matter. Throughout incubation, mineralisation remained slow
and weak, of co-metabolic type, and only 2% of the quantity applied was
mineralised over a period of 156 days. The amount of radioactivity leached varied considerably during incubation.
Under the effect of water percolation of 64 mm, the quantities exported in
equivalent 14C-phenanthrene went from 0.90 to 0.48 ng/g of soil between
0 and 35 days, then increased up to 1.5 ng/g at 100 days. The
radioactivity present in pore water displayed the same dynamic and the same
variations. For pore water, this radioactivity represented, in equivalent
phenanthrene, 0.09 ng/g of soil up to 46 days, then increased rapidly to
reach 0.5 ng/g at 65 days. In both cases (leachates and pore water),
enrichment in radioactivity due to the presence of a high proportion of
polar compounds reached 89%. Lastly, the formation of
non-extractable residues progressed very slowly up to 45 days (5.9 to 13%
of the amount applied), after which there was a marked increase, i.e. to
20.7, 32.2 and 35.8% at 52, 64 and 156 days, respectively. These
increases in radioactivity in different compartments of soil are correlated
with mineralisation of phenanthrene due to preliminary production of
degradation products. Thus, the capacity of a soil to degrade and mineralise
phenanthrene plays a determining role in the distribution of its residues
between the soil solution and the solid matrix. Overall, our results suggest
that in agricultural soils contaminated by strongly adsorbed organic
compounds, water pollution and sequestration are primarily controlled by
degradation.